This invention relates to an automatic transmission for an automotive vehicle having a feedback control function for supplying an adequate hydraulic pressure to frictional engaging elements of the automatic transmission during a shift from one speed ratio to another. More particularly, the present invention relates to a method of automatically correcting the initial hydraulic pressure to set it at a suitable value to be supplied to the frictional engaging elements of the automatic transmission even when the engine of the vehicle is replaced by another of different capacity such as an output torque or displacement different from that of the former.
In general, it is the function of an automatic transmission for an automotive vehicle that frictional engaging elements such as clutches and brakes are selectively actuated by application of a hydraulic pressure for stopping the rotation of a selected one of rotary elements relative to others in its gear system or connecting or disconnecting the rotary element with or from an input shaft, thereby automatically changing over or shifting the speed ratio depending on the operating condition of the vehicle. In order to protect the parts of the automatic transmission against damage or to ensure comfortable ride of occupants of the vehicle, the hydraulic pressure supplied to the frictional engaging elements of the automatic transmission is controlled gradually along a predetermined characteristic curve from the value of the initial hydraulic pressure supplied immediately after a shift to another speed ratio has started.
Nowaday, a variety of kinds of vehicles are sold on the market to meet a variety of needs, and a variety of kinds of engines are furnished to be mounted on such vehicles. However, due to the fact that the initial hydraulic pressure described above differs depending on the factor such as the displacement and/or output torque of engines mounted on vehicles, a variety of kinds of automatic transmissions must also be prepared to meet the variety of kinds of the engines. Suppose, for example, the case in which an automatic transmission adapted to be combined with an engine having a relatively large displacement is combined with an engine having a relatively small displacement. In such a case, since the operating hydraulic pressure (line pressure) of the automatic transmission adapted to be combined with a large-displacement engine is primarily high compared with that of the automatic transmission adapted to be combined with the small-displacement engine, the engaging force of the frictional engaging elements will become excessively large compared with the output torque of the engine, and the frictional engaging elements will be engaged by the initial hydraulic pressure as soon as a shift starting signal is generated, resulting in impartation of a great shift shock. In a converse case, on the other hand, the line pressure will be too low to start a speed shift or a large length of time will be required to attain the speed shift. Thus, it has been impossible to apply an automatic transmission of single capacity to an engine other than those of predetermined capacity.
Therefore, automatic transmission of a small number had to be manufactured for each of a variety of kinds, resulting in increased production costs and troublesome production control.